The standard for a Wireless Local Area Network (WLAN) technology is established by IEEE 802.11 standard association. IEEE 802.11a/b among IEEE 802.11 standards provides 11 Mbps (IEEE 802.11b) or 54 Mbps (IEEE 802.11a) transmission efficiency using unlicensed band on 2.4. GHz or 5 GHz frequency band. IEEE 802.11g, adapting OFDM (Orthogonal Frequency Divisional Multiplexing) technology, provides 54 Mbps transmission efficiency. And, IEEE 802.11n, adapting MIMO-OFDM technology, provides 300 Mbps transmission efficiency for 4 spatial streams. IEEE 802.11n provides 40 MHz channel bandwidth, and in this case it provides up to 600 Mbps transmission efficiency.
Now, a standard for regulating the WLAN operation in TV White Space is under establishment, as IEEE 802.11af.
TV Whitespace includes channels allocated to broadcast TV, which are permitted to be used by cognitive radio device. TV White Space may include UHF band and VHF band. The spectrum not used by a licensed device (hereinafter, can be called as ‘White Space’) can be used by an unlicensed device. The frequency band permitted to be used by unlicensed device can be differently defined for each country. Generally, this frequency band comprises 54-698 MHz (US, Korea), and some of this frequency band can't be used for the unlicensed device. Here, ‘licensed device’ means a device of the user permitted in this frequency band, and can be differently called as ‘primary user’, or ‘incumbent user’. Hereinafter, the term of ‘incumbent user’ can be collectively used for these terms.
512-608 MHz and 614-698 MHz are permitted to be used by all kinds of unlicensed devices except a few of special cases. However, 54-60 MHz, 76-88 MHz, 174-216 MHz and 470-512 MHz are permitted to be used by fixed devices, which perform transmission at a fixed location.
The unlicensed device, which wishes to use the TV White Space (TVWS), shall acquire information for available channel list at its location. Hereinafter, the unlicensed device operating in the TVWS using MAC (Medium Access Control) and PHY (Physical) operation according to IEEE 802.11 can be called as TVWS terminal.
Unlicensed device should provide a protection mechanism for the incumbent user. That is, the unlicensed device should stop using a specific channel, when an incumbent user, such as wireless microphone, is using that specific channel.
For this purpose, an unlicensed device shall access geo-location database through internet or dedicated network and acquire an available channel list at its location. The geo-location database stores and manages the information of registered licensed devices, location of the registered licensed devices and channel usage information, which is changed dynamically.
Or, an unlicensed device shall perform spectrum sensing. Spectrum sensing mechanism comprises Energy Detection scheme, Feature Detection scheme, etc. By using this mechanism, unlicensed device determines that the channel is used by an incumbent user, when the strength of the primary signal is greater than a predetermined level, or when DTV (Digital Television) Preamble is detected. And, the unlicensed device (station or Access Point) shall lower its transmission power, when it is detected that the neighboring channel, next to the channel used by the unlicensed device, is used by the incumbent user.
According to the related art, an unlicensed device must acquire a new available channel list whenever it moves more than a predetermined distance. Therefore, there is a problem that overhead for acquiring an available channel list is generated.